I may be totally, utterly and stupidly wrong of course. But an innocuous radio commercial I heard the other morning got me thinking, and my thoughts led to all sorts of tangles for the advertising industry as a whole. Let me explain.

I was in the car with my daughter on an environmentally-hostile school run. Now my daughter is a musical tyrant and will not listen to anything I want to listen to - the Today programme, my editor's podcasts - because she craves her daily dose of indie thrash.

So there we were in the car listening to her morning Xfm slice of Killers, Kooks, Kaisers and Klaxons when up popped a radio commercial for the mobile media company 3. And actually it was a very good commercial. It was about the idea of "new". "New rocks," said the chirpy voiceover. "People raise their eyebrows at new and say 'that's fantastic'." The ad went on to talk about the exclusive new handsets and new deals that 3 is dangling in front of consumers.

It is a very striking ad because it dares to flirt with the truth. The truth is that "new" is magical: we are drawn to "new" like moths to a light bulb; a shiny "new" MP3 player or digital camera or mobile phone is a way of saying that we're more in-the-know and more of-the-moment than the next person.

Because "new" is a materialistic truth it is obviously an advertising truth as well. The ad, made by 3's agency WCRS, is almost a piece of advertising satire.

There is a nice touch in the slightly ludicrous John Carpenter film They Live where the hero puts on a pair of glasses that allows him to see super-messages in the world around him. When he looks at poster sites, instead of seeing ads for washing powder and sweets, he sees words in big type like OBEY and THIS IS YOUR GOD. Those messages could have said THIS IS NEW. YOU WANT NEW. YOU WORSHIP NEW. Which is pretty much what the 3 ad does. Postmodern stuff. And thought-provoking stuff too. Because in the next few years, advertising will be put under new moral scrutiny for selling "new". Since the dawn of time, advertising has been slammed from socialist rag to soap-box as the cause of spiritual shallowness and personal debt, but now a new criticism of newmongering is starting to emerge - that "new" has a carbon cost too.

In extreme cases, it's environmentally good to replace old with new. I know someone with a 1979 Electrolux washing machine that still, somehow, works. The snag is, being a 1979 model, it is horrendously energy-inefficient and the quicker she changes it for a sexy new Zanussi A++, the better for the planet and all who live on it. But that is an extreme case. In most cases, replacing old with new is environmentally bad. Making new things produces carbon; disposing of old things produces carbon too. Changing your recently-cool Motorola Razr for a now-cool LG U400 is not cool in the slightest, not in the global warming sense.

Trouble is, it is what manufacturers want us to do. Manufacturers increasingly make money on production surplus and they pay their ad agencies to create desire for that surplus; to advertise MP3 players, digital cameras and mobile phones as must-have items of fashion; to make us all want more than we really need.

When the environmental excrement hits the fan, which I'm sure it will, what will advertising agencies do about it? If you were being optimistic, you would hope that the moral imperative and PR opportunity would encourage agencies to be brave. You would hope that an agency comes forward in the next year or two and insists that any advertising it runs has to include the carbon cost of that product. You would hope that an agency offsets its unsustainable desire-creation by creating a pro bono campaign to persuade people of the value of old. I would love to see an agency bid for the right to work on eBay, the world's principal pedlar of second-hand sustainability, on the condition that eBay gives that money to sustainable causes.

But if you were being pessimistic about an industry whose bean counters bang the loudest drums, you would expect the industry to do as little as possible. I've yet to hear of an agency turning down an airline account for ethical reasons; it's difficult to imagine an agency chief standing in front of an agency and proudly announcing that he or she has turned away the £50m 3 account on the grounds of need over want. WCRS, 3's agency, appends every email with an environmental caution against printing it. A nice touch, but whatever the industry does, it must do more than that.